Have you got an IR camera? If so, turn it on and have a look at each and every tantalum first. If not, have a look at each and every tantalum first :-)
Hmm interesting one. This is typical tek though.
If it’s not working in situ it’ll be in the feedback loop from the HV. On a couple of occasions, both 465b’s, I have broken that loop and hooked it to a bench supply to see if it had any kind of regulation. Turned out to be a resistor in one of the divider chains was out of spec so it stopped the whole oscillator dead.
“Working” is a very narrow margin between two posts of failure on these HT oscillators.
Edit: meant to say I like your HV probe. I bought a big Tenma “anal probe” out of fear in the end. I think yours is excellent for such low volume repair jobs. Maybe I need to cure the HV fear
Tektronix used more than 3 different 2N3055s and graded some of them. If you use the wrong one, then the high voltage oscillator may either not start or suffer from spurious oscillation.
The 151-0140-00 is the 0.3MHz low hfe variation. The closest modern part is the 2N3772G.
My Siemens 2N3055 seems to work all right. If they use a selected component in the high precision circuitry of a HP 3458A I'm all with them, but depending on it in a simple dc/dc converter doesn't get my blessings.
Quote from: David HessChecking the low voltages at the error amplifier is usually enough to tell what the inverter is trying to do and what the condition of the high voltage side is.
Sure. It tells me there is no high voltage. I still don't know why, however. I don't know how much my body's capacitive load was (and probe to fingers), but did it amount to more than, say 20 µA, of beam current?
Remember I isolated pins 8 and 9 already while it was still built in, and it looked totally overloaded. So I took a closer look at CR1512, C1512, CR1514 and C1514, but they all appear to be perfectly healthy. Judging from my external hookup, I'd say the transformer is also good, but with much less confidence.
The most common failure is a short in the high voltage multiplier. Next is a shorted high voltage decoupling capacitor. Unfortunately a low voltage test may not reveal either of these.
The high voltage multiplier can be tested simply by disconnecting either its power or ground inputs. The CRT will work without its PDA (post deflection acceleration) voltage applied although it will be dim, fuzzy, and have about half of its deflection sensitivity.
I always get suspicious if a secondary side short does not blow the primary side fuse and one of the things I would definitely do while diagnosing an apparent short in the high voltage inverter is replace the fuse with or add a constant current limit.
If the collector current is low with no output then there has to be something wrong on the primary side; the error amplifier should be pushing the 2N3055 hard producing a high collector current.
We might be oversimplifying the DC-DC as a one-transistor oscillator. Would Tek really use that here?
How Tek tuned this resonant to get sine verses a noisy flyback converter, I'm not sure.
For this circuit, there might be AC (ripple) as part of the feedback signal which helps the oscillator.
So not a strictly DC error voltage going into Q1484. Any open capacitor like C1455, C1472, C1494 could cause problems too.
I remember seeing a Tek service manual showing a shorted-turn on the HV DC-DC transformer to set the resonant frequency. I'll dig for that model number. You think that is the case here? I've never understood shorted turns, they always baffle me.
Ah yes, I found it- "flux balance winding" in the TM500 series (i.e. SC-502, SC-504). I'm mentioning it I thought it might be in a 464 as well, as the filament was later changed to DC power.
I think the only method left is to connect a signal generator to a winding and look at the waveforms.
Sphere has a couple 120-0909-01 transformers USD$65
I think if you ran it stuck at 0.45A for long time, it might have overheated the primary winding.
If you're sure the circuit has changed- you have the phasing correct still? Are you running it on the bench.
The feedback winding is biased at a negative voltage, so a lone 5k6 could be the problem for startup.
Transformers have several resonant frequencies and I'm surprised there's nothing to stop the leakage inductance from dominating.